Solar energy is one of Earth's largest potential sources of energy. Above the atmosphere, solar irradiance per unit area is 1.361 kilowatts per square meter. At sea level, the usable energy density is reduced to 250 watts per square meter. Using a two-dimensional model to approximate the Earth, 250 watts/square meter*π*6,371,000 meters2 yields about 32,000 terra (trillion) watts of energy that continuously strikes Earth's surface. Assuming the sun continues to burn and emit photons for a billion more years, the survival of human life ultimately depends on harnessing this essentially unlimited, source of clean energy.
The main impediment to widescale solar adoption thus far has been cost. Unlike other energy sources, solar energy costs are frontloaded while the operating costs are comparatively low. Fossil fuel-based energy sources require up-front costs as well as pay-as-you-go costs from consuming fuel. Unfortunately, not all the ongoing costs are reflected in the price of energy generated from fossil-fuel sources. These “dirty” energy sources have significant external costs stemming from CO2 emissions that, in the absence of a carbon tax, are not reflected in the cost. In addition to this cost advantage, entrenched utilities and fossil fuel producers have lobbied effectively to stymie the progress of solar, even in states with the greatest solar potential.
Notwithstanding these headwinds, the cost of solar has now dropped low enough that even when coupled with energy storage, solar power is equivalent to or less expensive than power generated from coal, oil and even natural gas. In the context of the electricity market, the relative cost difference between competing sources is quantified in terms of the cost per unit, typically a kilowatt hour (kWh). Large scale solar arrays, so called “utility-scale” arrays, may have tens to hundreds of megawatts of power generating capacity, putting them on the same scale as small coal and natural gas-fueled power plants. These arrays generate power that is fed into the grid and sold at wholesale prices on the order of a few cents per kWh.
The development of utility-scale solar projects is typically funded against power purchase agreements (PPAs). With a PPA, an off-taker (e.g., utility, grid operator, etc.) agrees to purchase all the power generated by the system at a fixed rate for the operational life of the array (e.g., 30 years). This enables a bank or other investor to accurately value the future revenue stream and to loan money against it to finance construction of the array.
Utility-scale solar power plants are predominantly configured as fixed-tilt ground mounted arrays or single-axis trackers. Fixed-tilt arrays are arranged in East-West oriented rows of panels tilted South at an angle dictated by the latitude of the array site—the further away from the equator, the steeper the tilt angle. By contrast, single-axis trackers are installed in North-South rows with the solar panels attached to a rotating axis called a torque tube that moves the panels from an East-facing orientation to a West-facing orientation throughout the course of each day, following the sun's progression through the sky. For purposes of this disclosure, both fixed-tilt and single-axis trackers are referred to collectively as axial solar arrays.
Excluding land acquisitions costs, overall costs for building utility-scale arrays include site preparation (road building, leveling, grid and water connections etc.), foundations, tracker or fixed-tilt hardware, solar panels, inverters and electrical connections (conduit, wiring, trenching, grid interface, etc.). Many of these have come down in price over the past few years due to ongoing innovation and economies of scale, however, one area that has been largely ignored is foundations. Foundations provide a uniform structural interface that couples the system to the ground. When installing a conventional single-axis tracker, after the site has been prepared, plumb monopiles are usually driven into the ground at regular intervals dictated by the tracker manufacturer and/or site plan; the tracker system components are subsequently attached to the head of those piles. Most often, these monopiles have an H-shaped profile, but they may also be C-shaped or even box-shaped. In conventional, large-scale single-axis tracker arrays, the procurement and construction of the foundations may represent up to 5-10 percent of the total system cost. Despite this relatively small share of the total cost, any savings in steel and labor associated with foundations will amount to a significant amount of money over a large portfolio of solar projects. Also, tracker development deals are often locked-in a year or more before the installation costs are actually incurred, so any post-deal foundation savings that can be realized will be on top of the profits already factored into calculations that supported the construction of the project.
One reason monopiles continue to dominate the market for single-axis tracker foundations is simplicity. It is relatively easy to drive monopiles into the ground along a straight line with existing technology even though their design is inherently wasteful. The physics of a monopile mandates that it be oversized because single structural members are not good at resisting bending forces. When used to support a single-axis tracker, the largest forces on the foundation are not from the weight of the components, but rather the combined lateral force of wind striking the solar panels. This lateral force gets translated into the foundation as a bending moment. The magnitude of the bending moment is much greater than the static loading attributable to the weight of the panels and tracker components. It acts like a lever arm trying to bend the pile, and the longer the lever arm, the greater the magnitude of the force. Many tracker companies specify a minimum foundation height of 40-inches or more. Therefore, in the context of single-axis trackers, monopile foundations must be oversized and driven deeply into the ground to withstand lateral loads.
One proposed alternative to monopile foundations is to use a pair of steeply angled legs to form an A-frame or truss-like foundation. An A-frame has the advantage of converting lateral loads into axial forces of tension and compression in the legs. As an example, this is seen in published U.S. Patent Application, 2018/0051915 (herein after, “the '915 application”). The '915 application teaches a support device for solar panels that consists of a pair of ground screws driven into the ground either parallel or at steep angles to one another and joined above ground with a bridge. According to the disclosure, in the angled embodiments, the legs are inclined towards one another at an angle that is preferably between 10 and 35-degrees, and more preferably between 15 and 25-degrees. That angle is the separation of the legs at the apex of the A-frame and corresponds to a leg angle in a range of ±72.5-degrees to ±85-degrees and more preferably ±78.5-degrees to ±82.5-degrees with respect to horizontal. As discussed in greater detail herein, such steep angles, while still capable of translating lateral loads into tension and compression, will result in tensile and compressive forces much larger than the underlying lateral load. The magnitude of the tensile and compressive forces generated by lateral loads is non-linearly correlated to leg angle, a fact that is not recognized by the teaching of the '915 application. As a result, at such steep angles, the legs must be oversized or include additional orthogonal features to resist the large values of tension and compression that are generated. Otherwise, the foundation will fail. This is part of the reason why ground screw-based A-frames have failed to gain traction in the utility scale solar industry, other than in the most difficult soils where costly refusals dominate.
In recognition of this problem, it is an object of various embodiments of this disclosure to provide a truss or A-frame foundation for single-axis trackers that is limited to a range of angles that reduces the non-linear magnitude of tensile and compressive forces imparted to the truss from lateral loads and thereby optimizes the amount of steel and depth of embedment needed for a given diameter leg.